Means for connecting devices of different impedance



May 12, 1925. 1,537,101

mums FOR commune DEVICES or DIFFERENT IMPEDANCE D. F. WHITING Rikin 1921 Patented May 12:, 1925.

UNITED [s ATEs- PATENT OFFICE.

DONALD E. WHITING, OF NEW YORK, N. Y., ASSIGNOR T WESTERN ELECTRIC COM- PANY, INCORPORATED, 013 NEW YORK, N. Y., A OORPORATION OF NEW YORK.

MEANS FOR CONNECTING DEVICES OF DIFFERENT IMPEDANCE.

Application filed .Tune .3 1921. Serial No. 474,650.

This invention relates to means for and methods of transmitting the energy of sig naling, ringing or other desired current The lnvent-ion relates more particularly to means whereby current of a desired he quency is transmitted with a high degree of efiiciency and a high degree of selectivity. These are simultaneously attained in trans mitting from 'a line or device of one impedance to a line or device of another impedance without the use of a transformer. In the preferred form ofthe invention use is made of anti-resonant or parallel-.

resonant circuits. If a device of acertain definite impedance is to be connected to a 95 circuit of Widely different impedance, it has been common to insert a transformer between the circuit and the device to step up or down the impedance of the device to equal that of the circuit to which itwas to be connected to secure the most efficient transfer of energy. If selectivity is desired in such an arrangement, it becomes necessary to insert tuning elements constituting a filter or equivalent selective circuits to transmit with, small attenuation the desired frequencies, and to highly attenuate the undesired frequencies.

. I In accordance with this invention, use is made of a parallel resonant circuit in which 40 the impedance element to be connected to a given circuit is placed. By adjusting the constants of the parallel resonant circuit,

its impedance at the selected frequency may be given any desired value within certain limits. The impedance of the parallel resonant circuit may thus be made equal to that of the connected circuit, thereby providing for maximum energy transfer from the given circuit to the parallel resonant circuit. Asthe addedreactance elements necessary,

, to properly proportion this latter circuit absorb negligible energy this means also a maximum transfer p of energy into the impedance element. The parallel resonant circuit acts also as a selective means.

If desired a series tuned circuit, or other tuning means may be added to provide additional selectivity.

Before describing certain practical applications of the invention, a brief theoretical discussion. will be given to make clear the principles involved.

Assume a parallel resonant circuit comprising a branch of negligible effective re sistance containing a capacity and another branch including a coil of inductance of the value L with an effective resistance .R.

Letting to stand for 21:), where fis the frequency, then the impedance of the inductive branch will be 'R+jo L, which Will be denoted z/A, that is, impedance of value 2 having 3 phase angle A. If ZQ designates the impedance of the two branches, when in parallel, then it can be shown that:

R iw[L CIR 11 00 ZLIPW The absolute value of this impedance 'is given by the equation,

The ratio Q of Z to 2 is given by the equation,

, 1 Q=JITETTWEW Difi'erentiating equation with respect to C as a variable and solvm Substituting the first value of 0 above, in equation 4, we get 7 given for a maxi- It will be evident that when equation (5) holds, the phase angle b=o-, hence Therefore, it is possible to step up the impedance of any particular coil, at a given frequency, to a value equal to the secant of the phase angle of the coil at that frequency by putting it in arallel with a theoretically perfect con enser of the proper value. At the maximum value of impedance the combined elements will act as an impedance comprising a non-inductive resistance.

. The application of the invention to practical arrangements is illustrated in the accompanying drawings, wherein Fig. 1 is a circuit diagram of a ringer controlling relay controlled from a circuit of much higher impedance, Fig. 1 is a modified arrangement, and Figs. 2 and 3 are circuit diagrams of a low frequency generator whose impedance is stepped up to a high value by means of a parallel resonant circuit.

, In Fig. 1 is illustrated an alternating current relay 1, which is to be controlled from a line circuit 3. The relay 1 in turn controls a ringing circuit 2. Relay 1 is operated by current of a given definite frequency, for example, 133 cycles, selected from the line 3, it being desired that currents of other frequencies on the line 3 will not cause the operation of the relay. The vacuum tube V serves to amplify currents from the line 3 and. transfer them to the relay 1. Let it be supposed that the imjustifiable assumption since commercial condensers are obtainable whose leakage is negligible at telephone frequencies and voltages. Assume, however, that the secant of the phase angle of the relay is 5, which corresponds to an impedance of 1,000 ohms, having a phase angle of 78 30'. Then the maximum impedance obtainable by shunting the relay with a condenser would be 5,000 ohms. In this case it is either necessary to redesign the relay to increase its phase angle or use some other expedient toaccomplish the desired result., If it is not desired to redesign the relay, and coils of a hlgh time constant are available, the

effective impedance of the relay may be stepped up to the required value by placing in series with it a suitable coil 5 and shunting the combination by suitable condenser 4. In the case assumed a suitable coil 5 will be one, which when placed in series with the relay 1, gives an impedance of such an angle and of such a value that the total impedance of the inductive branch multiplied by the secant of the phase angle equals 10,000 ohms, since in accordance with equation (6) a shunt condenser will raise the effective impedance to a value equal to that of the inductive branch of the circuit multiplied by the secant of the angle.

I R that when the relay 1 is shunted by a condenser 4 of such value as to produce a parallel resonant circuit having an impedance higher than 10,000 ohms, than it will be necessary, if this relay is to be used, to lace in series with it a capacity 18 of suc value as to reduce the effective induc- In case the time constant is so large tance of the relay. The capacity 18 should be substituted for the coil 5 and the whole combination shunted by a suitable capacity 4 as shown in Fig. 1.

Under some circumstances it may be desirable to insert a tuned parallel resonant circuit 6 across the output circuit of the tube for the. purpose of shunting disturbing frequencies. This may, however, be omitted. Likewise it may be advisable, in order to render the arrangement more selective, to

insert in series with circuit 1, 4, 5 a series resonant circuit. consisting of a condenser 7 and coil 8 adjusted to resonance at the frequency desired to be transmitted, which at the present instance is assumed to be 133 cvcles per second.

In Fig. 2 is illustrated the application of the invention to a mechanical generator. The microphone 'or tuned reed 9 is supplied with current from a source 1.1 in series with a highly inductive choke coil 12. The element 9 is actuated by an electro-magnet 10. A tuned resonant circuit 13, 14 is shunted across the combination of the elements 9 and 10. The circuit 13, 14 is tuned to the frequency of the waves to be generated, and the natural period of the element 9 is preferably the same frequency. If the generator 1 is to be connected to a line 15 having an impedance of 1,000 ohms a very inefiicient transfer of energy would result if the circuit 15 were connected directly across the terminals of the element 9, since the impedance of the element 9 in the ordinary case would be relatively low, for example,

about 50 ohms. By including the element v9 in circuit with a coil 16, having'a suitable time constant and proper value of impedance, and a' condenser 17, and by conmeeting the line 15 across the condenser 17, a more efi'icient transfer of energy will result, since the impedance of the parallel resonant circuit 9, 16, 17 as viewedfrom the line may be made equal to the im e'dance of the line. From equations 5, 6 an 7, it will be possible by a suitable numerical computation to determine the necessary constants of the coil 16 and the condenser 17, which will be required to secure the desired result.

The arrangement of Fig. 2 is the mos suitable when the device 9 is non-inductive or has an inductive reactance. If, however, the device 9 has a capacitative reactance, then the arrangement of Fig. 3 is more desirable. The arrangement of Fig. 3 differs from that of Fig. 2 in having the positions of the coil 16 and the capacity 17 reversed.

Thus by making the impedance in each direction as viewed from. the line A-A ment 9, w

equal to that in the other direction, a high ly efiicient transfer of energy will be effected. Furthermore the selective characteristics of the circuit 9, 16, 17 will cause a current havin a Ipurer wave form to be transmitted to t e me.

In connection with Fig. 3, it shouldbe noted that the equations,(1) to (7) given above are not applicable since in serles with the capacit 17 is the resistance of the elehich will be considerable, and must be taken into account. It will there fore be necessary toderive other equations from which the-constants of the various elements may be determined. By following the methods indicated'herein those skilled the art will have no difficulty in making the necessary computations for such cirsuits as Fig. 3. g

It has been assumed herein that the impedance of the output circuit of thetube V and that of the line 15 is non-reactive. In case there is a substantial departure from this condition, the im edance of the parallel resonant circuit shou d be made conjugate to that of the line by giving it such a value as to have an equal phase angle but of opposite sign. It will be seen that an arrangement of the kind described is in efi'ect a transformer since by its use .devices of dif ferent impedances may be connected so that energy will be transferred from one to the other without transmission or reflection loss due to their different impedances. The expression conjugate impedances is used in this s ecification to describe im-pedances havin t e relation (a-l-jb) and (a-jb).

aving described the invention in detail,

1. A translating device and a. circuit having an impedance different fromthat of the device, said circuit and device being connected for the interchange of energy of a given frequency therebetween, said device being included'in a parallel-resonant circuit to the terminals of which the said first mentioned circuit is connected, the impedance of said parallel-resonant circuit as viewed from the terminals of said first mentioned circuit being conjugate to the impedance of the first mentioned circuit.

2. A circuit arrangement for selectively transferring energy of a given frequency or a narrow band of frequencies from a line of one impedance to a device of another impedance, comprising a parallel-resonant circuit containing the given device connected across the terminals of the given line and containing resistance, inductance and capacity of such values, and connected in such branches of the circuit that the parallel resonant circuit presents an impedance of zero phase angle substantially equal to the impedance of the line at the given frequency or narrow band of the frequencies to be transmitted.

3. The combination of a parallel-resonant circuit connected to a given line including inductance and resistance in one branch 4. The combination of a line, a relay to I be operated by current of a dgiven frequency received from said line, sai relay being of an impedance much lower than that of the line, and impedance elements connected to said rela to form a parallel-resonant circuit, sai circuit being of impedance equal to that of the line.

5. In a combination according to claim 4, a series tuned circuit connecting the line to the cuit being resonant at the given frequency.

6. In an energy transferring circuit, an inductive translating device having a certain ratio of inductive reactance to resistance at a given frequency, and an inductive reactance in series therewith having a higher ratio of inductive reactance to resistance at the given frequency and a condenser shunted around the said device and the said reactance to form a parallel-resonant circuit, said condenser having a capacity of such value as to cause the parallel-resonant circuit to present a series impedance substantially equal to the im'pedanceof the inductive branchmultiplied by the secant of the phase angle of that branch in combination with parallel tuned circuit, said tuned cir a circuit connected across said condenser, the impedance of said circuit being substantially equal to said series impedance.

7. A circuit adapted to carry an alternating current and containin a resistance element, means for amplifying the apparent resistance of the element comprising an inductance in series withthe resistance and a capacity in shunt to these two.

8. The combination of a parallel resonant circuit, a second electrical circuit connected thereto a device having an impedance at a given irequency different from said second circuit and forming partof' said resonant circuit, the impedance of said resonant cir cuit at said given frequency being substantially equal to that of said second circuit.

' 9. An arrangement for connecting an electrical element of one impedance to an electrical circuit of another impedance in such a manner as to provide high eflicient energy transfer and a high degree of selectivity at a given frequency, which comprises a tuned loop resonant circuit including the given element and having its capacity, inductance, and resistance of such values that the impedance across the terminals of the tuned loop resonant circuit is substantially conjugate to the impedance of the circuit to which the device is to be connected.

In Witness whereof, I hereunto subscribe my name this 31st day of May A. D., 1921.

DONALD F. WHITING. 

